visual system Flashcards
main function of retina
Main function of the retina is image acquisition
Lateral Geniculate nucleus function
Lateral Geniculate nucleus preprocess the visual information
where does most processing of info occur
visual cortex
Two main cortical visual pathways
- ventral = object identification
- inferior temporal
- dorsal = object localisation
- posterior parietal
Pupil function
regulates the amount of light that falls on the retina
Lens function
focuses image on the fovea
Fovea function
a part of the retina with highest visual acuity
distribution of rods and cones
more cones at fovea, more rods at periphery of retina
structure of retina
pigment epithelium
photoreceptors
horizontal cells
bipolar cells
amacrine cells
ganglion cells -> optic nerve
neurotransmitter released by bipolar cells
glutamate
Feedforward neurons
photoreceptors, bipolar cells and ganglion cells
Feedback neurons
horizontal cells and amacrine cells
outer plexiform layer
synapses between photoreceptors, bipolar cells and horizontal cells
inner plexiform layer
synapses between bipolar, amacrine and ganglion cells
rod and cone response to light intensity
Rods are active at dim light and cones are active at bright
where does phototransduction occur
outer segment of retina
how do photoreceptors respond to light flashes
hyperpolarisation (decrease in membrane potential)
phototransduction cascade
- LIGHT = cGMP → GMP
- GMP → cGMP by phospodiesterase
- LIGHT = decreased glut
- DARK = increased glut
cGMP activates ion channels of membrane of photoreceptors
glutamate release in photoreceptors
Constant release of glutamate in darkness
Less glutamate is released during activation by light
laminar retinal organisation
- Bipolar and horizontal cells receive input from photoreceptors in the outer plexiform layer (OPL)
- Ganglion cells and amacrine cells receive input from bipolar cells (and ganglion cells from amacrine cells) in the inner plexiform layer (IPL)
ON cells response to light
ON cells depolarise when light intensity
OFF cells response to light
hyperpolarise in response to light
On bipolar cell activation
- Use mGluR instead of AMPAR
- G-protein (Go), not the same as in photoreceptor cascade
- Removal of cGMP is not required for ion channel closure
proteins with role in on cell regulation
TRPM1: expressed in ON but not OFF cells
Nyctalopin, proteoglycan required for light and glutamate responses in ON cells
on and off cell stratification in layers
- OFF = project closest layer to bipolar cells
- ON = project closest layer to ganglion cells
Axons of ON bipolar cells and dendrites of ON ganglion and amacrine cells are located in the same sublaminae in the IPL
Receptive field
= area in the retina (or space) which when illuminated activates a visual neuron
centre surround organisation
= organisation of bipolar(and gaglion) cells
- illumination of the center and the surround leads to responses in opposite polarities
- Some activate directly (centre) while others activate via horizontal cells(surround) and therefore cause the cell to depolarise
classes of ganglion cells
- parvocellular
- magnocellular
parvocellular ganglion cells
- are tuned to process information about shape and colour
- sustained response
magnocellular ganglion cells
- process information about motion
- fast responses
- transient response
morphology of ganglion cells dendritic trees
Morphology of ganglion cells differs in the size of the dendritic trees, some are assymetric, some are sparse and so on. Do not try to memorise each type. Just appreciate the diversity.
- E.G asymmetric dendritic trees
- useful directionality of motion
What is response adaptations and sensitivity
- Adaptation: decrease in activity (spiking rate)
- Sensitisation: increase in activity
